The present invention relates to ceramic bodies for use in automotive catalytic converter systems, and in particular to high porosity honeycomb substrates having reduced thermal mass and offering faster light-off performance.
The exhaust gases emitted by internal combustion systems utilizing hydrocarbon fuels, such as hydrocarbon gases, gasoline or diesel fuel, can cause serious pollution of the atmosphere. Among the many pollutants in these exhaust gases are hydrocarbons and oxygen-containing compounds, the latter including nitrogen oxides (NOx) and carbon monoxide (CO). The automotive industry has for many years attempted to reduce the quantities of pollutants from automobile engine systems, the first automobiles equipped with catalytic converter systems having been introduced in the mid 1970's.
Cordierite substrates, typically in the form of honeycomb structures, have long been preferred for use as substrates to support catalytically active components in part due to the high thermal shock resistance of such structures. Increasingly stringent regulations in recent years have been requiring higher conversion efficiencies in catalytic converters for the automotive market. Automobile emissions can be significantly reduced by controlling the light-off process. More than fifty percent of a vehicle's total emission is generated before light-off takes place.
The light-off time of a substrate is controlled to a large degree by its thermal mass; specifically, a lower thermal mass results in faster light-off times. One way to achieve a lower thermal mass is to reduce the thickness of the cell webs. Accordingly, demand for substrates having very thin webs (i.e., less than 2.5 mil (0.0025 inch, 0.0635 mm)) has dramatically increased. In addition to reduced mass, such structures may possess higher geometric surface areas, as well as lower backpressures.
Despite the advantages of thin-walled honeycomb structures, reducing the thickness of cell webs significantly diminishes the strength of the body. This causes problems during the canning process of the catalytic converter substrate. If the strength is decreased sufficiently, canning can induce a fracture of the substrate material. Further, manufacturing of such structures offers many challenges. Raw materials with smaller particle size and less contaminants are necessary to avoid screen and die plugging. Also, the extrusion pressure necessary to push the batch through a very narrow die slot may reach the limitation of the extruder.
An alternative way to reduce the thermal mass of a catalytic converter substrate is to increase the porosity while maintaining the cell density and wall thickness substantially constant. However, it is still important to maintain a low thermal expansion coefficient to prevent failures due to thermal shock. Unfortunately, high levels of porosity and low thermal expansion are known to reduce the strength of the honeycomb substrate.
Therefore, it would be considered an advancement in the art to provide a catalytic converter substrate with improved light-off performance by virtue of high porosity, while maintaining high strength and low thermal expansion, without the need for very thin cell webs. The present invention provides such a structure and method of making the same.